1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device capable of improving the brightness and display quality of a back light.
2. Prior Art
In a liquid crystal display device of active matrix type, non-linear devices (e.g., switching devices) are disposed in a manner to correspond to a plurality of pixel electrodes arranged in matrix, respectively. The liquid crystal in each pixel is always driven, in principle, (at a duty ratio of 1.0). In comparison with the so-called "simple matrix type" which employs a time division driving system, therefore, the active system has better contrast and has become an indispensable technique particularly in a color liquid crystal display device. A typical example of the switching devices is a thin film transistor (TFT).
A liquid crystal display panel (or a liquid crystal display device) is constructed: by superposing a lower substrate, which is formed, over a lower transparent glass substrate with reference to a liquid crystal layer, sequentially with a thin film transistor, a transparent pixel electrode, a passivation film for the thin film transistor, and a lower orientation film for orienting liquid crystal molecules; and an upper substrate, which is formed over an upper transparent glass substrate sequentially with a black matrix, a color filter, a passivation film for the color filter, a common transparent pixel electrode and an upper orientation film, such that their orientation films are opposed to each other; by bonding the two substrates by a sealing material arranged around the edges of the substrates; and by sealing up the liquid crystals between the two substrates. Incidentally, a back light is arranged at the side of the lower substrate.
Here, the liquid crystal display device of active matrix type using the thin film transistors is known in Japanese Patent Laid-Open No. 309921/1988 or on pp. 193 to 210 of Nikkei Electronics entitled "Active Matrix Type Color Liquid Crystal Display of 12.5 Type Adopting Redundant Construction" and issued on Dec. 15, 1986 by NIKKEI McGRAW-HILL, for example.